and it got me thinking: can this be modified or used to replicate a large gust of wind (think blizzard) knocking down a character?

I don't have access at the moment to the "x0_so_earthquake" script, but I don't remember there being anything specific to there being an Earthquake in the message to the PC, just "PC takes x damage from someone".

You do have access to the earthquake script - it's a standard Bioware thing. Goto your script editor, and select 'Open'. In the bottom left-hand corner, you should see three options - 'Module Resources', 'Hak resources' and 'All resources'. Choose 'All resources' and you'll be able to modify all of the standard scripts.

However, changing anything in a 'nw_s*' script will affect the magic system, so I wouldn't reccomend it - you're much better off writing a new script. I'll write something up later on when I'm more fully awake.